1. Field of the Invention
The present invention relates to data communication techniques and particularly to communication systems adapted to transfer data in a ring network, communication apparatuses forming a ring network, and method of monitoring a ring network.
2. Description of the Related Art
As a highly-reliable communication network, a ring network in which multiple communication apparatuses are connected in a ring topology is often built. The idea of “blockage” is introduced in a ring network in order to prevent a loop of communication frames. More specifically, a point where all communication frames are discarded (hereinafter, also referred to as “block point”) is provided in a ring network.    [patent document No. 1] 2002-271353
Known methods of verifying the normality (health) of a ring network include Ethernet OAM (“Ethernet” is a registered trade mark) standardized by the ITU-T Recommendation Y.1731 and IEEE802.1ag, or the use of a predetermined monitoring communication frame. All of the known methods are capable of verifying the normality only in a limited range, and the content of communication frames for verification is fixed. We considered that there is room for improvement in the precision of verifying the normality of a ring network.